Of optical switches using mirrors for switching optical paths, some use one-time reflection on a mirror and others use reflection twice before light from an input port is coupled to an output port. For example, from the viewpoint of a change in the polarization state of light due to reflection, a smaller number of times of reflection on a mirror is more preferable and a smaller incident angle relative to the mirror is more preferable.
FIG. 5A illustrates the configuration of patent literature 1 as a conventional example of a 1×2 optical switch. In FIG. 5A, reference numeral 11 denotes an input optical fiber and reference numerals 12 and 13 denote output optical fibers. Reference numeral 14 denotes a movable mirror and reference numeral 15 denotes a fixed mirror.
In this example, light from the input optical fiber 11 is reflected on the fixed mirror 15 and is optically coupled to the output optical fiber 13. Further, the movable mirror 14 moves to interrupt the optical path, so that the light from the input optical fiber 11 is reflected on the movable mirror 14 and is optically coupled to the output optical fiber 12. In other words, optical paths are switched by moving the movable mirror 14.
The movable mirror 14 is disposed on a driving part 16 shaped like a seesaw having a bar supported by a fulcrum, and the movable mirror 14 is moved in and out of the optical path by the rotation (seesaw movement) of the driving part 16. In FIG. 5A, reference numerals 17a and 17b denote electromagnetic coils for attracting the bar of the driving part 16. The bar is made of a soft magnetic material. Reference numeral 18 denotes a case.
FIG. 5B shows the detailed relationship between the mirrors and the optical paths of FIG. 5A. In FIG. 5B, αb denotes an incident angle (=reflection angle) of light relative to the movable mirror 14 and αc denotes an incident angle (=reflection angle) of light relative to the fixed mirror 15. In patent literature 1, the incident angles αb and αc are set at 20° or smaller, αb≠αc is set, and since there is a difference between the angles αb and αc, it is possible to prevent the output optical fibers 12 and 13 from overlapping each other. The specific numerical examples in Patent literature 1 are αb=8° and αc=13°.
In the above optical switch described in patent literature 1, the number of times of reflection on the mirror is one and the incident angle of light on the mirror is suppressed to 20° or smaller. In this respect, this optical switch is preferable because a change of the polarization state of light is small. However, this optical switch is characterized in that the incident angles of light relative to the two mirrors are changed, and thus the two outgoing light beams reflected and emitted from the two mirrors do not have the same polarization state, that is, the configuration of this optical switch is not suitable for a request to match the polarization states of the outgoing light beams with high accuracy.
In the 1×2 optical switch using the two mirrors, in order to accurately match the polarization states of the outgoing light beams which are switched and emitted, it is preferable to equalize the incident angles of light to the two mirrors. Patent literature 2 describes a mirror layout and an optical path configuration for equalizing the incident angles of light to two mirrors.
However, in the configuration of patent literature 2, the two mirrors cannot be placed close to each other on an optical path and a large clearance is necessary between the two mirrors. This is because two output ports are disposed on the same side with respect to an input port. This point will be specifically described below with reference to FIG. 6. In FIG. 6, the input port 11 and the two output ports 12 and 13 are optical fibers and the optical fibers 11, 12, 13 and two mirrors 14 and 15 are indicated by the same reference numerals as those in FIG. 5A.
Two mirrors 14 and 15 are disposed in parallel with each other to equalize the incident angles of light to the mirrors 14 and 15, and two output optical fibers 12 and 13 have optical axes disposed in parallel with each other. In this case, the following relationship is established:EF·sin ∠FEG=FG where E and F represent the reflection points of light on the mirrors 14 and 15, the light being incident from an input optical fiber 11, and FG represents a perpendicular line from a point F to the optical axis of an output optical fiber 12.
When light to the mirrors 14 and 15 has an incident angle of, for example, 10°, ∠FEG=20° is established. Further, when the output optical fibers 12 and 13 are φ125 μm in diameter (clad diameter) and a distance between the optical axes of the output optical fibers 12 and 13 is 125 μm which is equal to the clad diameter, FG=125 μm is established and a distance EF between the mirrors on an optical path is determined by the formula below:EF=125/sin 20°=365 μm
Such a large clearance between the two mirrors 14 and 15 causes the following problem: In the case where the input optical fiber 11 has, for example, a rod lens on the end of the fiber and has a light-gathering function, the position of a beam waist (a position where a beam diameter is minimized, that is, the focal position of a lens provided on the end of the optical fiber) formed by light from the input optical fiber 11 is aligned with one of the two mirrors 14 and 15 and is considerably separated away from the other. For example, when the position of the beam waist is aligned with the mirror 15, the mirror 14 is irradiated with a larger light spot and thus the area and the driving stroke of the mirror 14 inevitably increase. On the other hand, when the position of the beam waist is aligned with the mirror 14, the mirror 15 is irradiated with a larger light spot and thus the area of the mirror 15 inevitably increases.
Such an increase in mirror area causes a serious problem in an optical switch of, for example, a MEMS (Micro Electro Mechanical System). In other words, it is necessary to increase the thickness of a silicon layer (silicon device layer) making up the mirror and accurately etch the thick silicon device layer in a perpendicular direction. Further, an increase in driving stroke causes a problem of an increase in driving voltage required for an actuator.
Patent literature 1: Japanese Patent Application Laid-Open No. 2003-248180
Patent literature 2: Japanese Patent Application Laid-Open No. H01-306811